The present invention relates to probe stations adapted for measuring the performance of integrated circuits and other devices under varying temperature conditions.
Integrated circuits (ICs) are manufactured on semiconductor wafers that can include many individual devices or electronic circuits, known as dies. Individual dies are electrically tested with probes that are connected to test instruments and brought into contact with test pads on the die. This typically occurs at a probe station which includes a stage supporting a wafer chuck having a top surface on which a wafer can be restrained during probing. Typically, a vacuum system is connected to the chuck and selectively connectable through passages or tubes, internal to the chuck, to several openings in the top surface of the chuck. A wafer, resting on the top surface of the chuck and covering the openings, is secured by air pressure when a vacuum source is connected to the openings. The stage supporting the chuck is generally movable to position the wafer under one or more probes that can be brought into contact the test pads on the wafer. The test instrumentation applies predetermined electrical excitation through the probes to certain test points and senses the response of the respective circuit or device to the excitation.
Measuring performance under variable temperature conditions is a substantial component of IC performance testing. An IC may be tested at elevated or depressed temperatures to determine the potential effect on performance of operating the IC at its limits or using or storing an electronic device incorporating the IC in various environments. Typically, a thermal chuck is used to heat or cool the wafer in preparation for probe testing at a non-ambient temperature. The top surface of a thermal chuck typically comprises a thermal plate which is usually cast or fabricated from aluminum, another metal, or a non-metal that exhibits good thermal conductivity and dimensional stability over the range of temperatures at which testing will be performed. The simplest form of thermal chuck incorporates a heater element that heats the surface of the thermal plate supporting the wafer. Heaters can take several forms, such as plate heaters, coil heaters, mica heaters, thin film heaters, or heater rods incorporated into the chuck or cast into the chuck's structure. To cool the wafer to a temperature below the ambient temperature, a thermal chuck may also include passageways for the circulation of a cooled liquid or gas. On the other hand, thermal chucks may incorporate thermoelectric devices that can alternately heat or cool the wafer. The thermoelectric devices are based on the Peltier effect where heat is released or absorbed at a junction of two dissimilar semiconductors when current flows through the junction. Since the direction of heat flow at the junction is determined by the direction of current flow in the junction, by reversing the direction of current flow a single device can be used to, alternatively, heat or cool the chuck.
When a wafer or other structure to be tested is placed on a relatively hotter or colder surface of a thermal chuck, a heat flux is induced over a substantial portion of the contacting surfaces and heat is, respectively, absorbed from or conducted to the chuck. Heat transfer to or from the wafer continues, elevating or depressing the temperature of the wafer, until the temperatures of the contacting surfaces equalize. Once the mass of the wafer has obtained a stable temperature, testing can be performed on the individual circuits making up the wafer. While generalized heating and cooling of the mass of the wafer with a thermal chuck produces the elevated or depressed temperatures desired for various testing regimes, the process is time consuming because the mass of the chuck and the wafer must be heated or cooled to modify the temperature of the individual dies that are to be tested. What is desired, therefore, is a method and apparatus for reducing the time required to test individual dies of a wafer at temperatures above or below the ambient temperature.